Compression of digital speech and audio signals is well known. Compression is generally required to efficiently transmit signals over a communications channel, or to store said compressed signals on a digital media device, such as a solid-state memory device or computer hard disk. Although there exist many compression (or “coding”) techniques, one method that has remained very popular for digital speech coding is known as Code Excited Linear Prediction (CELP), which is one of a family of “analysis-by-synthesis” coding algorithms. Analysis-by-synthesis generally refers to a coding process by which multiple parameters of a digital model are used to synthesize a set of candidate signals that are compared to an input signal and analyzed for distortion. A set of parameters that yield the lowest distortion is then either transmitted or stored, and eventually used to reconstruct an estimate of the original input signal. CELP is a particular analysis-by-synthesis method that uses one or more codebooks that each essentially comprises sets of code-vectors that are retrieved from the codebook in response to a codebook index.
In modern CELP coders, there is a problem with maintaining high quality speech reproduction. The problem originates since there are too few bits available to appropriately model the “excitation” sequences or “codevectors” which are used as the stimulus to a synthesis filter. An improved method for determining the codebook related parameters has been described in U.S. patent application Ser. No. 11/383,506, filed on the same date as this application and is incorporated herein by reference. This method addresses a low complexity, joint optimization process and method. However, there remains a need for improving performance of CELP type speech coders at low bit rates.